The effects of the convulsant and anticonvulsant barbiturates, 5(2-cyclohexylidine ethyl)-5-ethyl barbituric acid (CHEB) and phenobarbital (PHB) were studied at the frog neuromuscular junction. Spontaneous transmitter liberation was augmented irrespective of the external Ca 2 ion concentration by both drugs, but CHEB was considerably more active than PHB. Miniature end plate potential amplitude was equally reduced by both drugs. The net effect of these agents on end plate potential (EPP) amplitude varied depending on the external Ca 2 ion concentration. In Ca 2 ion deficient Ringer's solution both drugs at 1 x 10 the minus 5 power M increased the quantal content and the epp amplitude (CHEB much more than PHB). In Ringer's solution containing a normal concentration of Ca 2 ions, both drugs depressed the evoked response and failed to later the quantal content significantly,